1. Field of the Invention
The present invention relates to a navigation device which is adapted for use in a pre-determined geographic area. More particularly, the invention relates to a navigation device using a magnet which is free to rotate so that it can align with the earth""s magnetic field.
2. Description of the Related Art
A conventional compass usually includes a magnetic needle which is mounted for rotation over a face indicating the principal directions of north, south, east and west at ninety degree intervals, and may include gradations therebetween for precise determination of a traveler""s direction or bearing. The needle is typically journaled on a pin fixed in a housing having a transparent cover for viewing the position of the needle relative to the compass face, whereby the user can determine which way is north. When a user wishes to orient himself with respect to features on a map, the map must be oriented with respect to the compass, so that the north point on the map corresponds to the direction of north as indicated by the compass.
While the foregoing procedure might be considered great fin by someone on an orienteering expedition in a remote area, it is somewhat burdensome for a tourist in an urban area, who typically is not armed with a compass and may not know how to use one anyhow. Emerging on the street above a subway station in a strange city with only a map, he or she might have no clue how to orient the map to find a destination. Furthermore, he or she typically does not care which way is north; he or she only wants to know which way to head to reach the destination. He or she therefore needs a simple navigation device for determining the right direction. Such a device would also be useful for someone already familiar with a city, since emerging from an unfamiliar subway station can be disorienting.
According to the invention, a magnet is mounted for rotation with respect to a base so that the magnet tends to align with the earth""s magnet north, as in a conventional compass. However the magnet is provided with a display surface, which is preferably on a disc fixed to the magnet and freely rotatable therewith, the display surface having map indicia for a predetermined geographic area. The map indicia are oriented to define a direction of magnetic north with respect to the map indicia, as are any at least two points on a map. The map indicia on the display surface are also aligned with respect to the magnet so that the direction of magnetic north with respect to the map indicia coincides with the earth""s magnetic north when the magnet is aligned with the earth""s magnetic north. However the direction of magnetic north need not be indicated on the disc bearing the map indicia.
The base, the magnet, the disc, and a transparent cover are preferably assembled as a compass module, which in turn can be fixed to a housing having a slot for storing a map. According to a preferred embodiment, the map may be partially withdrawn from the slot and unfolded for use with the compass, while remaining fixed to the housing. The housing can be rotated until the map indicia on the disc have respective orientations which are parallel to the corresponding indicia on the map. This is done in the same fashion as rotating a compass housing so that the north point on the face is aligned with the north end of the needle. The difference is, that no additional step of aligning with a map is necessary; this is accomplished in a single step by rotating the housing to which the map is fixed.
The compass module may be designed according to one of several embodiments including a first embodiment using an elongate neodymium or samarium cobalt permanent magnet and a ferrofluid. A ferrofluid is a fluid that can be manipulated by a magnetic field. The ferrofluid used in the compass module is a stable, colloidal suspension of nano-sized particles in a liquid carrier, the particles being coated with a surfactant to prevent agglomeration. In the presence of a magnetic field, the magnetic moments of the particles align themselves with the field lines, allowing the fluid to behave as a homogenous liquid. The magnetic field also increases the density of the fluid. When applied to the magnet, the fluid forms beads at opposite ends of the magnet, which beads support the magnet as a low friction bearing and also act as a damper to minimize oscillations.
According to a second embodiment the magnet is held in a carrier fixed to the disc by means of a bracket extending through an aperture in the disc and retained by the magnet itself. The bracket has a recess which is centered on a protrusion on the base. The recess is preferably a conical recess and the protrusion is preferably a cone which is self-centering in the recess.
The map indicia selected for representation on the disc will generally be well known indicia for the pre-determined geographic area for which the device is designed. In Manhattan, for example, the disc may be labeled with quadrants representing uptown, downtown, the east side, and the west side. But note that the orientation of these indicia with respect to the magnet does not correspond to north, south, east, and west when the magnet is aligned with magnetic north, because the direction of Uptown is 30 degrees toward the northeast. Likewise the Westside is angularly displaced counterclockwise by thirty degrees from west. The indicia chosen need not correspond to sections of the city but may be roads, a river, or landmarks. The only important thing is that someone with only limited familiarity with the area will recognize the indicia so that he can orient himself in the geographic area. By aligning a map with the indicia on the disc, he can also orient himself to features shown in greater detail on the map.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.